1. Field of the Invention
This invention relates generally to methods of treating positive photoresist layers and more particularly relates to treatments of alkali soluble resin-diazo ketone photoresists which result in changing the developed resist profile. The invention is particularly useful in forming an undercut resist profile with normal optical exposure of a photoresist but is not limited thereto. Undercut resist profiles are needed, for example, in various microcircuit fabrication processes, especially when employing the lift-off technique.
2. Description of the Prior Art
An additive metallization technique, known as the "lift-off" technique, was developed with the advent of electron beam lithography. This technique utilizes the fact that electron scattering in a resist layer and backscattering from the underlying substrate create a pear-shaped energy absorption profile in the resist layer which results in an undercut profile after resist development. With an undercut in the resist image, metal may be evaporated over the entire resist surface and discontinuities will remain between the metal deposited through the resist openings onto the underlying substrate and the metal deposited on the resist top surface. When the resist layer then is removed with a suitable solvent, the metal on the resist is also removed and a clean and faithful reproduction of the image is obtained on the substrate in metal. This lift-off technique also may be used to form multilevel metal structures and any material or combination of materials which may be deposited by evaporation may be used instead of metal.
Undercut resist profiles may be easily obtained when exposing with an electron beam because the energy absorption in the resist layer during exposure is not linear but reaches a maximum at about two-thirds of the beam penetration range. In optical exposure of photoresists, however, energy absorption is highest at the top of the resist layer and lowest at the interface between the resist and the underlying substrate due to light attenuation in the resist. Standing wave effects created by light reflecting back from the substrate further complicate the situation. It is thus ordinarily impossible to obtain an undercut profile or even a vertical profile with normal optical exposure and normal development of the commonly used positive photoresists, such as the alkali soluble resin-diazo ketone photoresists.
A technique that has been used in electron beam lithography to increase sensitivity while maintaining an undercut profile consists of coating the substrate with two or more resists having widely different solubilities in a developer. The top resist layer is chosen so that it develops much more slowly than the underlying layer(s) in order to produce an undercut profile. U.S. Pat. No. 3,934,057 describes such a technique in more detail. This technique cannot be applied well, however, to alkali resin-diazo ketone photoresists because it is too difficult to coat two distinct layers of this resist type without excessive mixing at the interface even when the first layer is baked at the maximum allowable temperature before applying the second coat.
A similar two-layer technique using photoresist is described in an article entitled "Double-Resist Layer Shadow Masking" by T. Gardner et al, IBM Technical Disclosure Bulletin, Vol. 19, No. 1, June 1976, pp. 156-7. In this method a first layer of positive photoresist is deposited, dried and uniformly exposed. Then a second layer of positive photoresist is deposited, dried and pattern exposed. An undercut profile is produced upon development because the lower layer was exposed everywhere. This technique has the disadvantage of requiring two separate exposure steps with a resist deposition step in between. Furthermore, the lower layer will dissolve when applying the upper layer with many commonly used photoresist compositions including the alkali soluble resin-diazo ketone resists.
Another two-layer technique using photoresist is described, for example, in Applied Physics letters, Vol. 31, No. 5, September 1977, pp. 337-9, and entitled "Offset Masks for Lift-off Photoprocessing." In this method in order to prevent the bottom photoresist layer from becoming dissolved during deposition of the top photoresist layer, a thin intermediate metallic layer is deposited to protect the lower resist layer during deposition of the top photoresist layer. The top layer is developed and used as a mask for etching through the intermediate metallic layer. Then the intermediate metallic layer and overlaying top resist layer act as a mask during development of the bottom resist layer. This involves still more additional processing steps.
Techniques are also known for treating the surface of a single layer of alkali soluble resin-diazo ketone photoresists so that an undercut profile develops. U.S. Pat. No. 4,007,047 describes a process wherein the photoresist layer is treated with hydrogen ion by dipping the layer in an acid bath. This is said to decarboxylate the exposed sensitizer molecules. The present invention also involves the step of treating the resist surface with a liquid, namely a solvent, but the solvents used are substantially pH neutral.
In an article entitled "Single layer Optical Lift-off Process" by R. Bergin et al, IBM Technical Disclosure Bulletin, Vol. 18, No. 5, October 1975, p. 1395, a technique is described for obtaining an undercut profile in a phenolformaldehyde diazo ketone resist layer by treating the layer with a solution of a thermal free radical initiator so that cross-linking occurs in the upper regions of the layer. The solvents used with the present technique do not initiate cross-linking in the resist.
In another article entitled "Single Coat Photoresist Lift-off Structure" by C. Johnson et al, IBM Technical Disclosure Bulletin, Vol. 19, No. 3, August 1976, p. 859, a technique is described for obtaining an overhang or undercut profile in phenolformaldehyde diazo ketone resist by treating the resist layer with an oxygen plasma. The solvents used in the present technique do not oxidize the resist surface.
It should be noted that all of these photoresist surface treatment techniques cause a molecular modification of the resist material or a component thereof in the region near the top surface so as to make this region less soluble thereby producing an undercut profile. The solvents used in the present invention are not believed to be chemically reactive with any of the resist constituents, so that resist treatment does not modify or change the structure of molecules in the resist material.
U.S. Pat. No. 3,961,100 describes another resist surface treatment which increases the effective electron beam sensitivity of a diazo ketone resist. In this process an electron beam sensitive diazo ketone resist film is contacted with a developer solution, washed with water and dried prior to exposure to an electron beam. While the mechanism of this treatment was not described, it was speculated that the developer solution increases the wettability of the resist film such that when thereafter contacted with water, a "surface reaction" occurs to form a "skin" on the surface of the resist film.
The present invention pertains to light exposing of resist films rather than electron beam exposing thereof and does not employ a treating solution which chemically attacks or reacts with the resist, which developer solutions will do by definition.